1. Field of the Invention
The present invention relates to a photodetector in which the wavelength multiplexed light is selected every wavelength region and each of the selected in lights is converted into the electric signal which is outputted. Such a photodetector can be suitably used for a receiver or the like in a wavelength multiplex optical communication.
2. Related Background Art
FIG. 1 is a schematic perspective view showing a structure of a conventional photodetector of the wavelength selective type.
A buffer layer 102 and a slab type optical waveguide 103 are formed on a substrate 101. An edge surface of an optical fiber 106 is coupled to the optical waveguide 103. Photodetectors 104.sub.1 to 104.sub.3 and gratings 105.sub.1 to 105.sub.3 are formed on the upper surface of the optical waveguide 103. The optical fiber 106 guides the wavelength multiplexed light signal. The signal light is input to the slab type optical waveguide 103 by the edge surface coupling.
The gratings 105.sub.1 to 105.sub.3 as reflective type filters which are arranged like an array in a cascade form diffract only the lights of the wavelengths which satisfy the Bragg conditions which are determined by an incident angle and a period among the incident lights. The lights selected in accordance with the wavelength regions as mentioned above are respectively photoelectrically converted by the photodetectors 104.sub.1 to 104.sub.3 provided for the gratings and are output as electric signals.
However, the photodetector as mentioned above has the following problems.
(I) In the case of selecting the light of a short wavelength, the period of the gratings must be set to a very small value and it is difficult to manufacture such gratings. PA0 (II) Since a whole length of the gratings needs to be set to a value within a range from 100 .mu.m to hundreds of .mu.m, there is a limitation when the photodetector is miniaturized. PA0 (III) Since the gratings have a two-dimensional extent, the matching with a three-dimensional (channel type) waveguide is not good.